1. Field of Invention
This invention relates to lasers. Specifically, the present invention relates to multiple core laser systems and related phase-locking techniques.
2. Description of the Related Art
Lasers are employed in various demanding applications including fiber optic telecommunications, laser surgery, bar code scanning, compact disk readers, and military targeting and tracking. Such applications often demand compact, high-power, eye-safe lasers.
Small, high-power, eye-safe lasers are particularly important in military airborne applications, including airborne ladar, range finding, target identification, and missile guidance, which often involve large standoff distances. Missile and aircraft space constraints necessitate particularly compact lasers. Unfortunately, conventional eye-safe lasers are often too bulky and under powered for many military applications.
Eye-safe laser systems typically either emit directly at eye-safe wavelengths or generate beams that are frequency-converted to eye-safe wavelengths between 1.4 and 1.8 microns. Lasers that emit directly at eye-safe wavelengths often employ glass hosts, which severely limit thermal handling, power scalability, and overall laser system applicability.
To enhance power output, Nd-doped YAG crystal (Y3Al5O12) lasers are often employed. Nd:YAG lasers typically employ frequency conversions, such as Raman and optical parametric oscillators (OPO's) to convert an intermediate beam into an eye-safe beam operating between 1.4 and 1.8 micron wavelengths. These systems require several relatively large pump sources and bulky conversion optics and are not readily scalable to high average power or high pulse power. Consequently, Nd:YAG lasers employing conventional frequency-conversion systems are impractical for many applications.
Alternatively, combining multiple laser beams via phase locking based on the Talbot effect may increase power scalability. However, phase-locking systems based on the Talbot effect generally require precise periodical structures of equal lengths to facilitate phase locking. Phase locking of Multiple Core (multicore) Fiber (MCF) lasers via the Talbot effect generally requires individual equal-length fiber cores and is highly dependent on the individual power output in each core. Unequal length fibers may yield prohibitively inefficient lasers. Furthermore, the fibers require precise periodic positioning. Mechanical and thermal manufacturing issues necessitate expensive processing for precise fiber length equalization and periodic fiber positioning, which may yield prohibitively expensive lasers.
Furthermore, the number of elements that can be phase locked via the Talbot effect is generally limited by common cladding multiplexing issues. Consequently, power-scalability is limited. To achieve multi-kilowatt powers, several multicore fiber lasers would be required. However, the Talbot effect is insufficient to effectively phase lock and thereby coherently combine several multicore fiber laser oscillators.
Alternatively, a digital control method is employed to phase lock multicore fiber outputs. The digital control method involves measuring phases of beams output from individual multicore fibers and then providing feedback to the multicore fiber pump sources to align output phases to facilitate beam combining, i.e., phase locking. Unfortunately, such phase-control techniques often require complicated and expensive digital control loops and beam phase measuring equipment.
Hence, a need exists in the art for an eye-safe, high-quality, robust, cost-effective, compact, and light-weight laser that is readily scalable to high average power and high pulse energy. There exists a further need for a unique phase-locking system that can efficiently combine arbitrary numbers of fiber laser oscillator outputs without requiring precise fiber length equalization or stringent periodic positioning.